1. Field of the Invention
The present invention relates generally to seismic sensors which are deployed within a fluid filled borehole of a well to monitor seismic waves. More particularly, the present invention relates to a method and apparatus suitable for coupling geophone sensors to a borehole wall in high temperature and pressure environments.
2. State of the Art
In borehole seismology, motion sensors are lowered into the borehole of a well to monitor seismic waves emitted from a seismic source placed within the borehole or at surface locations proximate thereto. The emitted seismic waves travel through the earth surrounding the borehole and under certain conditions are reflected and/or refracted by subterranean formations or variations in the surrounding strata. By recording the reflected and refracted seismic waves with the sensors it is possible to map the structural and compositional properties of the earth around the borehole. Such information is valuable, by way of example, to locate and determine the characteristics of oil and gas reservoirs during energy exploration.
In a typical configuration, an array of sensor modules containing geophone type sensors is lowered into the borehole on a cable called a wireline, also sometimes referred to as a logging cable. Alternatively, a tubing string may be used to deploy the array. The geophones operate via a component that measures displacement between a stationary first part and a second part mounted for movement along an axis in response to vibrations from the seismic waves. Often, the geophones are constructed as 3-component, or triaxial, sensors which are arranged to record in the vertical (Z) direction, as well as first and second (X and Y) horizontal directions, providing a reading for each of the three orthogonal components of the seismic waves. Due to the displacement measuring technique by which geophones sense seismic waves, a firm, uninterruptible interface between the geophones and the transmission medium for the seismic waves is required to receive the vibrations. Optimal performance may be accomplished by clamping or forcing a sensor module against a wall of a borehole to provide an improved mechanical coupling for conducting seismic waves to the associated geophone.
Various techniques have been used in the prior art in an attempt to maintain contact of a sensor module with a borehole wall. In one approach, extendable mechanical arms are incorporated into the sensor module. When the sensor module is positioned at a desired location within the borehole, the arms are extended from the module body to press against one or more surfaces of the borehole wall and clamp the sensing portion of the module against an opposite surface. For boreholes lined with metallic casings, as is often the situation with oil and gas wells, magnetic means have also been used to attach a sensor module to the borehole wall. Such systems are usually operated from a location, such as on a drilling rig floor, above the earth's surface and involve complicated attachment mechanisms that are sometimes incapable of effectively clamping in borehole regions having irregular shapes or surface topographies. Sensor modules of this type may also be mechanically complex, expensive to construct and add substantial extra weight, which must be carried by the wireline. As a sensor array may contain dozens or even hundreds of sensor modules the practical application for such costly and heavy devices is somewhat limited.
Another conventional coupling technique involves using an inflatable bladder or “packer” that is expanded in a borehole to force an associated sensor module into contact with a wall. U.S. Pat. No. 6,206,133 to Paulsson, U.S. Pat. No. 5,111,903 to Meynier and U.S. Pat. No. 5,027,918 to Cole disclosed common examples of inflatable bladder type sensor modules. While this inflatable bladder coupling approach is desirable in terms of construction, weight, and clamping versatility, the bladder structures raise other concerns with respect to performance under the hostile conditions frequently encountered within the confines of a deep well borehole. Typically, the bladders involved are formed of an elastomeric material such as a rubber, polyurethane or vinyl composition, and may be reinforced with one or more layers of flexible fabric such as polyester or nylon. The bladders are inflated with fluid under pressure which is supplied by tubing extending from the surface or from fluid reservoirs located on the sensor array. At great depths, especially when the fluid contained in the borehole comprises a dense slurry of particulates in a water or hydrocarbon-based drilling fluid (often referred to as “mud”), hydrostatic pressures may approach or even exceed levels of 25,000 psi. This environment requires that the inflation fluid pressure for the bladders be carefully controlled to assure adequate expansion without over-pressurization, which may burst the bladders. Furthermore, depending on the type of fluids residing within a borehole and the depth at which a sensor array is located, ambient temperatures may reach 500° F. (260° C.) or greater. At these temperature levels, the above-described bladder materials may be substantially damaged or degraded, rendering the bladders useless. Further, conventional bladders, particularly those located at substantial depths, are inflated by downhole pumping systems that commonly include a motor in a sealed housing filled with a nonconductive fluid and pressurized using a bladder or diaphragm. This approach equalizes pressure between the motor armature and borehole fluid, allowing the motor to drive an impeller with a shaft extending through a dynamic seal for pumping high pressure borehole fluid. The sealing of the motor and shaft is susceptible to leakage, and dynamic fluid pressures acting on the motor cause power losses and limit functionality in terms of speed and brush float, if the motor is so equipped.
As is evident from the foregoing description of the state of the art, a technique is needed for coupling sensors such as geophones to a borehole wall that overcomes the structural and durability problems associated with conventional approaches. More particularly, what is needed is a coupling system for use with sensor modules that is suitable for use in the extreme pressure and temperature conditions of a deep well borehole.